I kiss my girlfriend's twin sister, what should I do?

Last week I went to a party with my friends, my girlfriend and her sister.

I was a bit drunk and I start grabing and kissing my girlfriend, after 5min I realized that it was her twin sister. She didn't say anything to stop me and she was turned on!

My girlfriend didn't see us, so I was wondering, should I tell her? And what is wrong with her sister, what should I do with her now?
I kiss my girlfriend's twin sister, what should I do?
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